CD4 and graft versus host disease: This study demonstrate that: i) HCMV infection is much more frequent in seropositive than in seronegative patients; ii) as a consequence, the virus-specific T-cell response was much more frequent in HCMV-seropositive patients; iii) however, protective activity of the T-cell response against HCMV infection was detected in both seropositive and seronegative patients; iv) protection was stable and long-lasting, unless steroid therapy for GvHD was administered; v) both HCMV-specific CD4+ and CD8+ T-cells are required to confer protection against HCMV reactivation.